1. Field of the Invention
The present invention relates generally to spectrometry, such as emission, absorption or transmission spectroscopy, and particularly to a spectrometer, which can be realized as microspectrometer.
2. Description of the Related Art
With the spectrometer, it is possible to measure light in a certain spectral range in a wavelength-dependent way. Center piece of every spectrometer is a dispersive element, such as a grating or prism, which light enters, whose spectral distribution is to be determined, and which decomposes the incident light into its spectral components, and a corresponding detector for sensing one or several of the spectral components. FIG. 3 shows a conventional arrangement of a grating spectrometer. A moveable grating is illuminated by an entrance slit and a collimating element (not shown) with a light beam 902, whose spectral distribution is to be determined. The movable grating 900 is rotatably mounted, wherein the adjustment of the grating 900 is performed quasi-statically, typically via a step motor. The light 906 split into wavelengths—specifically, a spectral component of the same—is detected by detector element 908, while the grating 900 is moved into different positions. In that way, the light 906 split into wavelengths is sampled by the detector 908, wherein its measurement signals are shot correspondingly to determine the spectral distribution of the light beam 902.
For so-called low-cost applications, devices of the type of FIG. 3 are too expensive and too costly due to their expensive mechanical control of the quasi-static movement of the grating 900. Additionally, the type of device of FIG. 3 is sensitive against shock and temperature variations and the light proofness of the housing (not shown) of these devices has to be ensured despite all required device feeding lines, for, for example, the mechanical drive of the grating 900 and the operation of the detector element 908. Particularly, the measurement length for determining the spectral distribution of the incident light 902 is too long.
Apart from the conventional solutions of FIG. 3, miniature spectrometers in the form of PC plug in cards or in the form of smaller external housings with a corresponding computer interface have existed for several years. The basic setup of these miniature spectrometers is shown in FIG. 4. A grating 920, to which the light beam 902 enters, a photodiode line 922 as well as a required control logic (not shown) is disposed on a board (not shown). In these miniature spectrometers, the grating 920 is disposed fixed or pinned and is in an encapsulation (not shown), together with the photodiode line 922. An example for a miniature spectrometer of the type shown in FIG. 4 is, for example, shown in DE 19836595A1.
Although the prices of such spectrometers with fixed grating are low, it is a disadvantage of these fixed grating systems, that the wavelength range detected by the photodiode line 922 as well as the spectral resolution are set during production and are thus invariable. One possibility to adapt wavelength range and spectral resolution to a particular application based on an existing supply of such fixed grating spectrometers, is merely the usage of several spectrometers in parallel in a master-slave operation, wherein several spectrometers of different resolution and different wavelength ranges, respectively, are coupled. Thereby, however, additional costs per slave module occur. Additionally, a flexible solution where the detectable wavelength range as well as the spectral resolution can be changed any time is also not obtained by this modular coupling of several modules.
One specific problem in the design of miniature spectrometers is that the available space is limited, whereby, on the one hand, the detectable detection range and, on the other hand, the obtainable resolution is limited. The detectable wavelength range depends on the fixed grating dimensions, particularly the grating distance and the distance of the grating from the detector element, the so-called base length. Apart from other amounts, such as the amount of an entrance or exit slit and the utilized refractive order, the resolution of the spectrometer depends on the base length, the grating number and the distance of the photodiode elements of the photodiode line and the exactness of the grating positioning, respectively. If, for example, by using a photodiode line with fixed density and by considering maximum dimensions, a spectrometer of the type of FIG. 4 is designed for high resolution, this is performed at the cost of the detectable spectral range, and conversely, if the spectrometer is designed for a large spectral range, a poorer resolution results.
Typical process spectrometers, which are formed in the form of two-line spectrometers with fixed grating and CCD or photodiode line, use line photo detectors with typically 1024 to maximum 2048 lines. For separating two spectral lines, theoretically, at least three line elements are required, wherein, however, practically about 5 to 7 lines are required for separating spectral lines. Assuming the usage of an optical fiber as a slit and the base length of a PC card spectrometer, a resolution in the range of 0.2 nm in a spectral detection range of 60 nm results with these line spectrometers. This resolution is sufficient for many applications, but the resulting spectral detection range is too small for many applications, since, for evaluation, usually two or more spectral lines of the spectrum to be examined have to be taken into consideration and thus have to be within the detection range. Alternatively, if the spectrometer is designed for a lower resolution, a larger range of, for example, 600 nm can be achieved, such that it would be sufficient for the spectroscopy across the visible spectral range, i.e. for the wavelengths of 400 nm to 800 nm, but the resulting resolution of 2 nm provides only a very poor quality.
As the previous discussion has shown, there is a need for a spectrometer type which can be produced micromechanically, has a high mechanical stability and sufficient properties both with regard to the spectral detection range and the spectral resolution and can at the same time be produced at low cost. In the above conventional spectrometer solution of FIG. 3, there are problems in a miniaturization with regard to the producible deflections of the grating, which again limits the spectral detection range. Further, the expensive control for the quasi-static movement prevents a cost-effective realization. In the fixed grating systems of FIG. 4, no high resolution can be realized with a simultaneous large spectral detection range.